Bremsstrahlung Measurements Research Articles

Bremsstrahlung diagnostics of hot electrons in laser-plasma interactions

Hot electrons produced in the ultra-intense laser-plasma interactions play very important roles in the scheme of “fast ignition”. Bremsstrahlung measurements can be an effective method to diagnose the hot electrons accelerated in the forward direction. The transport of the hot electrons and the characteristics of the bremsstrahlung photons were calculated using the Monte Carlo electron-photon transport code MCNP. The feasibility of the bremsstrahlung diagnostic method in laser plasma interactions is also discussed.

Comparisons of electron energy deposition derived from observations of lower thermospheric nitric oxide and from X-ray bremsstrahlung measurements

[1] Previous studies have shown the connection between electron precipitation and the excess amounts of nitric oxide at auroral latitudes. In this study the electron energy deposition derived from thermospheric nitric oxide (NO) measurements is compared with the electron energy deposition derived from X-ray bremsstrahlung measurements. The electron energy deposition is derived from nitric oxide densities by use of a photochemical model for nitric oxide and is referred to as the modeled energy deposition. The comparisons are made for the beginning of five geomagnetic storms in 1998: 21 March, 2 May, 14 June, 26 June, and 16 July. By using these quite different methods to derive the total electron energy deposition (4–100 keV), the results show that we have a generally good understanding of the physics and chemistry of the energy transfer from electron precipitation in the lower thermosphere. The comparisons also show some discrepancies. The modeled energy deposition is typically larger than the energy deposition derived from X-ray bremsstrahlung in the beginning of the storm period, whereas later on in the storm the energy deposition derived from X-ray measurements is largest. The cases where the modeled energy deposition is largest is probably due to production of NO occurring before the bremsstrahlung measurements. The systematic underestimate of the calculated energy deposition could be due to uncertainties in the reaction rates or in the characteristic electron energy used in the photochemical model. The effects from horizontal neutral wind on the NO gas from the production on the nightside to the observations on the dayside can also be a source of discrepancy.

Evidence of the influence of reflections on the Zeff profile measurements in the Tore Supra tokamak and their mitigation

Reliable experimental determination of the mean effective charge (Zeff) of the plasma is of enormous importance for the impurity control in high temperature plasmas, allowing the performance of various discharge scenarios to be assessed and the benchmarking of model calculations, ideally accompanied by measurements of the Zeff profile. The Zeff is usually calculated using data from line-integrated visible bremsstrahlung measurements, electron density and temperature profiles, and the plasma geometry. On Tore Supra (TS) it had been noticed in the past that hot spots on the inner wall, MARFEs or supra thermal electrons can falsify the signal of the visible bremsstrahlung. More significant, however, was a systematic overestimation of the edge channels, giving unreasonably high edge Zeff values of 30 or even higher. The analysis of dedicated discharges in the present configuration of TS leads us to conclude that this is due to the influence of vessel wall reflections. A newly developed inversion method which takes reflections into account shows that the presence of reflections leads to an overestimation of the outer channels in the reconstruction of the Zeff profile. This is due to the larger relative contribution of the reflections on the edge channels due to the shorter path length through the plasma and the lower bremsstrahlung emission at the edge. We present a model which takes into account this effect and allows a correction procedure to be applied when calculating the Zeff profile. The TS data can be modeled assuming Lambertian (diffusive) reflections with a reflectivity coefficient of 20%–25%, which is in agreement with measurements of the reflectivity of the inner wall of the TS vacuum chamber.

A new method on recycling coefficient measurement using impurity pellet injection in a large helical device

Recycling coefficients of carbon, aluminum, and titanium were evaluated using a new technique combining impurity pellet injection with high-spatial resolution bremsstrahlung measurement in hydrogen and helium plasmas on the large helical device. The recycling coefficient of impurities was investigated by measuring absolute intensities with the visible bremsstrahlung array. The time evolution of the bremsstrahlung signals was modeled by an impurity transport code adjusting the diffusion coefficient, convective velocity, and recycling coefficient. As a result, a finite value of the recycling coefficient was required in the case of carbon, whereas aluminum and titanium were explained as nonrecycled particles. It was also clarified that the recycling coefficient of carbon had a larger value in hydrogen plasmas (R=0.5–0.65) than in helium plasmas (R=0–0.2), suggesting the formation of hydrogen molecules.

P3 Measurement of bremsstrahlung as a practical alternative to the use of a liquid scintillation counter for the assessment of 90Y activity

Royal United Hospital, Bath, UK Autumn Meeting of the British Nuclear Medicine Society: abstracts York, UK, 4–5 October 2004

Bayesian analysis of the effective charge from spectroscopic bremsstrahlung measurement in fusion plasmas

At the stellarator Wendelstein 7-AS (W7-AS) a spectrally resolving two channel system for the measurement of line-of-sight averaged Zeff values has been tested in preparation for its planned installation as a multichannel Zeff-profile measurement system on the stellarator Wendelstein 7-X (W7-X) which is presently under construction. The measurement is performed using the bremsstrahlung intensity in the wavelength region of ultraviolet to near infrared. The spectrally resolved measurement allows to eliminate signal contamination by line radiation. For statistical data analysis a procedure based on Bayesian probability theory has been developed. With this method it is possible to estimate the bremsstrahlung background in the measured signal and its error without the necessity to fit the spectral lines. For evaluation of the random error in Zeff the signal noise has been investigated. Furthermore, the linearity and behavior of the charge-coupled device detector at saturation has been analyzed.

Z eff from spectroscopic bremsstrahlung measurements at ASDEX Upgrade and JET

The effective ionic charge Zeff is a means to assess the impurity content of a fusion plasma. It can be derived from measurements of bremsstrahlung intensity. These have been extended at ASDEX Upgrade by the usage of the sight lines for the charge exchange recombination diagnostic. Together with a previously installed sight line array, it is now possible to routinely determine the bremsstrahlung intensity over the whole minor radius purely from spectroscopic measurements. In a tokamak where the plasma facing components are made up of various materials, this is necessary to check if measurements are contaminated by line radiation. The bremsstrahlung background of the respective spectra is determined using Bayesian probability theory, giving consistent and improved error statistics. Using the information for electron temperature and density profiles, the Zeff profile is determined by an integrated method. The same approach to assess the Zeff profile has been demonstrated to be successful also at the JET tokamak.

A study of charge dependence of particle transport using impurity pellet injection and high-spatial resolution bremsstrahlung measurement on the Large Helical Device

The spatial structure and Z dependence of transport coefficient were studied by injecting impurity pellets (C, Al, and Ti) and measuring high-spatial resolution bremsstrahlung in steady-state neutral beam heated plasmas on the Large Helical Device [A. Iiyoshi et al., Fusion Techol. 17, 169 (1990)]. The D and V were evaluated from a diffusive/convective model with minimizing the residual error between the experimental and computational bremsstrahlung intensities taking the recycling rate into account. As a result, the existence of an inward convection was observed at ρ>0.6, and no convection was required at ρ<0.6. Furthermore, it is found that the dependence of D on the electron density, the species and charge state of impurity is weak [typically D=0.15–0.25 (m2/s) in the range of ne=1.4–5.2×1019 (m−3)], but the inward convection has a strong dependence not only on the electron density but also the charge state. The spatial structure and Z dependence of the convective velocity for carbon and titanium ions were evaluated.

Direct removal of edge-localized pollutant emission in a near-infrared bremsstrahlung measurement

Visible and near-infrared electron–ion bremsstrahlung measurements in fusion research devices, used to determine the effective ionic charge (Zeff), are often plagued by pollutant emission from the cool-edge region. The primary sources of visible and near-infrared non-bremsstrahlung continuum emission in the Madison Symmetric Torus arise from electron–neutral interactions, and the pollutant emission is hence directly proportional to the bulk neutral density. Simultaneously monitoring the total emission at 1040 nm and the neutral contaminant (via Dα emission) has enabled an extraction of the electron–ion bremsstrahlung and a measurement of Zeff at comparatively low electron density (1013 cm−3).

Hot-Electron Plasma Studies in the Plug Section of the Hanbit Tandem Mirror

Hot electrons have been created in the plug section of the Hanbit tandem mirror in order to allow a test of high-in ballooning stability provided by a high-β hot-electron plasma in a tandem mirror. A rectangular microwave cavity was built to confine the energy from a 2-kW 14-GHz klystron. The cavity was equipped with a diamagnetic loop, a skimmer probe, and bremsstrahlung windows. An end-loss probe has been added in the cusp section in order to study the hot-electron mirror losses from the plug. The end-loss probe contains a Silicon PIN diode that is used to detect the x-rays from fast electrons striking a tantalum radiator. The end-loss probe was scanned radially to determine the radius and radial width of the hot-electron distribution ring for two different magnetic fields. A clear ring is observed for both magnetic fields. Bremsstrahlung measurements have shown the presence of a hot-electron plasma in the plug with an electron temperature in the range of 60 to 120 keV. The temperature with the optimum magnetic field is ~ 100 keV. Diamagnetic measurements give the total stored energy. Stored-energy measurements combined with the radial dimensions determined by the end-loss detector were used to give the value of beta with assumptions on the plasma length. The average beta value is much less than 1% due to the low power and short heating time.

Solar flare electron acceleration: Comparing theories and observations

A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration

Solar flare electron acceleration: Comparing theories and observations

A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration.

An accurate absolute-scale measurement of bremsstrahlung following absorption of incident X- and γ-rays

Measurement has been made of the spectrum of bremsstrahlung due to photoelectrons ejected by incident photons of energy 59.5 keV. Use was made of a coincidence set up with two HP germanium detectors, the target-detector recording the energy of the ejected photoelectron after it radiated a bremsstrahlung photon and a second detector measuring the energy of the radiated bremsstrahlung photon. Detailed analysis of the applied experimental method was undertaken. Also included in this report are details of the processing of the data, including calculations of various detector-to-detector cross-talk processes. Experimental values of cross section are in good agreement with calculated values obtained using either the well-known semi-empirical thick-target formula or a simple theoretical model of bremsstrahlung radiation due to photoelectrons in an infinitely thick target.

Consistency check of Zeff measurements in ergodic divertor plasmas on Tore Supra

On Tore Supra, the mean effective charge Z eff of the plasma is deduced routinely from line integrated measurements of visible Bremsstrahlung. In order to get a deeper understanding of this measurement, and to obtain high confidence in the data, we have compared the visible Bremsstrahlung data to line radiation of impurities present in the plasma, removing any error from the processing technique. The paper presents a systematic study of the correlation between the Z eff derived from the two methods, and the dependence on plasma parameters and geometrical factors. A good correlation is found, independent of the position of the outboard pump limiter (OPL). However, the fitting parameter of the empirical method is different for different plasma currents, due to the variation of the emission profiles. The agreement between the two methods is good, confirming the role of carbon and oxygen as the main impurities in the Tore Supra tokamak.

A model for low‐energy thick‐target bremsstrahlung produced in a scanning electron microscope

AbstractThick‐target bremsstrahlung measurements were obtained for various atomic numbers and for energies between 10 and 25 keV. A Jeol JSM 6100 scanning electron microscope was used for the electron beam and a Oxford Instruments Link Si(Li) detector for the bremsstrahlung spectrum. The experimental data were compared with a model that computes the thick‐target bremsstrahlung spectrum by integrating the tabulated relativistic partial‐wave doubly differential cross‐section for thin‐target bremsstrahlung over the target taking electron energy loss into account in the continuous slowing down approximation. The model corrects for electron backscattering, x‐ray attenuation in the target and the efficiency of the x‐ray detector. Very good agreement was found between the model and the experimental results. Extension of the model to consider multiple scattering in the target is also discussed. Copyright © 2001 John Wiley & Sons, Ltd.

Quantifying baryon stopping in high energy nuclear collisions

We propose a numerical definition for baryon stopping in relativistic heavy ion collisions that is obtainable from final hadron rapidity distributions as well as from bremsstrahlung measurements. Thus a new channel of communication is opened between the two methods.

High-Precision Proton-Proton Bremsstrahlung Measurements below the Pion-Production Threshold

An exclusive proton-proton bremsstrahlung measurement has been performed with polarized protons of 190 MeV. Absolute cross sections and analyzing powers have been measured with unprecedented accuracy in a large part of the phase space. The data are compared with state-of-the-art theoretical calculations including higher-order off-shell effects, like the $\ensuremath{\Delta}$ isobar and meson-exchange currents. Surprisingly, the calculations are unable to describe the data in detail.

Probing few-body systems with bremsstrahlung

A series of bremsstrahlung measurements have been performed at KVI with the 190 MeV polarized proton beam of the new superconducting cyclotron, AGOR. The aim of these measurements is to investigate the dynamics of interactions involving intermediate off-shell nucleons. Nucleon-nucleon bremsstrahlung is the most fundamental process used for such studies as it involves the strong interaction between two nucleons and the well-known electromagnetic interaction. The present experiment provides the most accurate data on the proton-proton system. In addition the first exclusive proton-proton virtual-bremsstrahlung data ever is presented. The accuracy of the real bremsstrahlung data is high enough to investigate higher-order effects such as the virtual Δ-isobar and meson-exchange currents. None of the microscopic calculations is able to describe the data both in magnitude and in shape. The virtual-bremsstrahlung data are integrated over the detector acceptance. This same integration is done for two different soft-photon calculations for comparison.

A fast programmable multiplicity trigger system

A fast, programmable trigger module has been designed and built. It consists of commercially available components mounted on two multilayer boards and resides in a CAMAC environment. The module has been designed to provide a first-level trigger primarily for the detection of proton pairs from proton–proton bremsstrahlung measurements. The design considerations and the performance of the module are discussed.

Implications of high-energy photons and electrons on target preheat at the Sandia “Z” facility

High-energy photons and electrons on the Sandia National Laboratories “Z” accelerator, a z-pinch device, will deposit energy into a capsule and fuel; this may create a potential preheat problem for inertial confinement infusion (ICF). In this article we discuss heating of the capsule and fuel by high-energy photons and electrons. The fuel is heated to <2 eV, in a time-integrated sense, on Z by these particles. Because peak implosion occurs at the peak in the soft x-ray emission on Z, the heating at times of interest is reduced roughly an order of magnitude to ∼0.2 eV for times of interest and fuel preheat from this mechanism is concluded to be small. These estimates are generated from time-integrated bremsstrahlung measurements. The uncertainty in the heating is high because the electron spectrum is not known directly, but inferred. In addition, the influence of photons and electrons at energies between 5 and 60 keV is not known. Given the uncertainties at this point, we do not know the impact on the feasibility of internal dynamic hohlraums for a z-pinch-driven ICF implosion. We discuss these issues and suggest directions for further study.